The present invention relates to material handling and more particularly to a delivery control system for a vehicle-mounted spreader.
In color climates it is a common practice to spread sand, salt/sand mixtures or heat-producing bulk chemicals onto icy or snowpacked streets and highways. Spreading trucks used for this task generally include a bulk material storage hopper, a belt or screw type conveyor for delivering the bulk material to be spread from the storage hopper to the rear of the truck and a spinner for spreading the delivered material over a path of predetermined width. The amount of bulk material spread is, generally speaking, a function of the conveyor speed. The spinner speed, which typically remains constant, controls only the width of the covered path.
Ideally, the bulk material is to be spread at a uniform rate since too much material is wasteful while too little material does not substantially improve roadway conditions. The latter situation is particularly dangerous since a driver who has been lulled into complacency by sections of properly treated roadway may not be alert enough to avoid an accident when an inadequately treated section of roadway is unexpectedly encountered.
Hydraulic control systems have been developed to control the delivery of bulk materials. A typical hydraulic control system includes a servo controlled variable volume pump driven by the truck engine. The hydraulic pump feeds a hydraulic motor which drives the conveyor at a reasonably constant rate. The conveyor speed is fed back in a closed loop around the pump/motor combination to regulate out disturbances that might otherwise be caused by instantaneous variations in truck engine speed. The intended result is to control the conveyor speed to produce a desired material spread rate in material weight per unit of time; for example, in pounds per hour.
An immediately apparent drawback to this system is that the desired spread rate is based on an assumption that the spreader truck can maintain a desired constant speed, an objective which can rarely be met in view of the conditions under which spreader trucks operate. A truck driver who travels at a faster than assumed speed must either change the reference input to the hydraulic system or ignore the fact that too little material is being spread. The converse is true. A slower-than-assumed truck will spread too much material unless the driver makes the proper adjustments. Truck speed seldom can be kept constant and a conscientious driver will be forced to make adjustments constantly. Even if the driver is willing to do this, the results are seldom as uniform as might be desired.
Hydraulic control systems are believed to be available in which road speed is taken into account by using a speed transducer to adjust a flow valve at the pump output. The response time of such a system may be relatively slow since the hydraulic motor will require some time to respond to changes in input flow.
Hydraulic spreader control systems have other disadvantages. The components are costly by themselves. The systems are costly to install and, because of the corrosive nature of many of the bulk materials being spread, costly to maintain.
It has been suggested that a hydraulic control system might be replaced by an electric control system including a solid state inverter which would convert 12 volt battery power to a controllable d.c. voltage to be supplied to a direct current drive motor for a conveyor.
A primary drawback to such a system is cost. Because of battery drain, a special heavy duty truck generator is necessary, as is a heavy duty battery. A solid state inverter is also relatively expensive. The accumulated costs of these components is undesirably high. Moreover, the proposed inverter is a constant speed system which does not take variations in road speed into account.